A known form of reflectors for light are prisms and particularly triple prisms which, in ideal conditions, reflect the incident light from any given direction in a parallel beam. Generally, triple prisms consist of a cylindrical body one end of which forms a point of three surfaces disposed at a mutual angle of 90°. In other words, one end of the cylinder is in the form of a triangular pyramid. Light entering the cylinder through the plane surface at the other end thereof is reflected in parallel to the entering beam but in the opposite direction by total reflection on the surfaces of the pyramid. Of course, this reflection process requires that the triple prism consists of a material having a higher refractive index than the environment.
Possible applications of such prisms are e.g. in simulation systems for military training, identification of friend or foe in planes, but also in other applications where the presence of an object is to be detected and the object is to be identified. To achieve a large range, laser beams are used which scan the environment, and a beam reflected by a triple prism is detected by a sensor located near the light source.
An aim of such installations is to detect not only the presence of an object but also its identity. One possibility consists in permanently monitoring the position of each object by means of a supervising control unit. If an object is detected in a given position, the control unit can determine the identity of the object through its knowledge of the positions of all objects. The disadvantage of this approach is that it requires a complete surveillance of all objects by a central unit, thereby creating high demands with respect to the corresponding interlinking and a considerable delay in the detection.
Another possibility is that the reflector modulates the reflected light beam, thereby returning information related to the object equipped with the triple prism to the emitter. The result is a substantial reduction in complexity, a simpler structure of the entire system as the moving objects are autonomous, and a more rapid identification of the located objects.
A reflector of this kind is described in U.S. Pat. No. 4,143,263. According to this reference, an optical switch is disposed in front of the reflector, e.g. a liquid crystal element, a piezoceramic modulator, or a KDP cell (KDP: potassium dihydrogen phosphate). U.S. Pat. No. 4,249,265 suggests a mechanical solution using a shutter in front of the reflector.
However, the mentioned solutions suffer from different disadvantages: they are either expensive, provide a limited modulation depth, or attenuate the light beam.